Typically, at least with shows here in the US, it's an actor that used to be a reasonably big star or is well known. Usually a bit part in terms of overall plot, but is stuck in there as a sort of actor/actress that gives the show legitimacy.
You can't help but watch the Patriots; even if you aren't a fan of the team. It's pure entertainment, I'd go as far to say as much entertainment as the St Louis Rams when they were scoring 40 points a game and 500 yards worth of offense.
The game last night was a good one, I thought it was all...
Three cheers to Dave the Gaff though, thought your comments were spot on. I left for lunch after that, shame that there weren't more Brighton and Hove Albion calls taken.